(a) Technical Field of Invention
This invention relates to an inkable sheet, and, in particular, to a sheet suitable for use with a mechanical printing assembly, such as an ink jet printer or a pen plotter.
(b) Background of the Art
With the recent proliferation of micro-computers and colour monitors there has been a massive growth in the amount of information available for display in colour. Presentation of such information has created a demand for hard copy, for example--on paper sheets, but increasingly on transparent polymeric films which are capable of serving as imaged transparencies for viewing in a transmission mode. Preparation of the desired hard copy is conveniently effected by, for example, an ink jet printer or a pen plotter, using an aqueous or an aqueous-organic solvent-based ink.
Ink jet printing is already established as a technique for printing variable information such as address labels, multi-colour graphic, and the like. A simple form of ink jet printer comprises a capillary tube coupled to an ink reservoir and a piezo-electric element which, on application of a voltage pulse, ejects an ink droplet from the capillary tube at high velocity (e.g. up to 20 ms.sup.-1) onto an ink-receptive sheet. Movement of the ink jet may be computer controlled, and new characters may therefore be formed and printed at electronic speeds. To derive advantage from this high speed operating capability requires the use of an ink-receptive sheet which will quickly absorb the high velocity ink droplet without blotting or bleeding. Although plastics sheets may be employed, these generally tend to exhibit inferior ink absorption and retention characteristics. In particular, drying of an applied ink pattern is slow, and immediate handling of a freshly imaged sheet is therefore prevented.
Pen plotter assemblies are extensively used in drawing offices, and particularly in the generation of computer aided designs. The advent of polymeric recording sheets has revealed that the formation thereon of inked images of acceptable quality usually requires the development of special, and expensive, pens. However, an inkable sheet according to the invention permits the use of a simple, inexpensive, fibre-tipped, aqueous ink, pen of the kind hitherto conventionally used with paper recording sheets.
(c) The Prior Art
Various recording sheets have been proposed for use with ink jet printers. For example, British patent specification GB No. 2050866-A discloses an ink-jet recording sheet comprising a layer of a water-soluble coating polymer disposed on a support having a water absorptivity of not more than 30 gm.sup.-2 (JIS P8140). The support, which may be of paper, cloth, plastic film, metal sheet, wood board or glass sheet, should be sized, if necessary, to provide the specified water absorptivity level to prevent penetration of the water-soluble coating polymer into the support. The characteristic feature of the sheet is that the layer of water-soluble polymer, which desirably has a high viscosity, dissolves or swells in the water of a subsequently applied aqueous ink to increase the viscosity of the ink. Although such behaviour is said to provide an image of high density, high resolution and good colour reproduction without causing ink overflow, mixing or flying, a pattern applied to the sheet using an aqueous organic solvent-based ink is relatively slow to dry.
British patent specification GB No. 2116880-A relates to a material, used to bear writing or printing, comprising a substrate having a coating layer which is divided by micro-cracks of irregular form into lamellae. The width of each micro-crack is usually several microns, whereby the solvent medium of a subsequently applied ink passes through the micro-cracks and is quickly absorbed into the substrate which comprises a porous, liquid-absorbing material, such as paper. The dimensions of the micro-cracks are such that the product is opaque and therefore unsuitable for use in the production of transparencies for use in an overhead projector. Furthermore, the requirement for the solvent medium of the ink to pass through the micro-cracked structure into the porous substrate precludes the provision of an intermediate adhesive layer to promote adhesion between the substrate and coating layer.
We have now devised an inkable sheet which is particularly suitable as a recording sheet for use with a mechanical printing assembly, such as an ink jet printer or a pen plotter.